Magnetoresistive Random Access Memory (MRAM) has a read function based on a tunneling magnetoresistive (TMR) effect in a MTJ stack wherein a tunnel barrier is formed between a free layer and a reference layer. The free layer serves as a sensing layer by switching the direction of its magnetic moment in response to external fields (media field) while the reference layer has a fixed magnetic moment.
MRAM devices comprised of MTJ elements wherein one or both of the free layer and reference layer have PMA are preferred over their counterparts that employ in-plane anisotropy because the former has an advantage in a lower writing current for the same thermal stability, and better scalability. In MTJs with PMA, the free layer, which stores information for the memory bit, has two preferred magnetization orientations that are perpendicular to the physical plane of the layer. Without external influence, the magnetization direction of the free layer will align to one of the two preferred directions, either up or down, representing information “1” or “0” in the binary system. For memory applications, the free layer magnetization direction is expected to be maintained during a read operation and idle, but change to the opposite direction during a write operation if the new information to store differs from its current memory state. The ability to maintain free layer magnetization direction during an idle period is called data retention or thermal stability and has a different requirement for each memory application. A typical non-volatile memory device may require thermal stability at an elevated temperature of 125° C. for approximately 10 years.
For conventional CoFeB based free layers, PMA originates from the interface between the CoFeB free layer and MgO tunnel barrier. However, the physical shape of the free layer, having a lateral dimension more than ten times the thickness, tends to induce in-plane anisotropy wherein the free layer (FL) magnetization direction will be in the plane of the FL. Moreover, as FL thickness (volume) increases, a greater portion of the free layer is away from the FL/tunnel barrier interface where PMA is generated. Accordingly, PMA is further reduced until at a sufficiently large FL thickness value, FL anisotropy is entirely “in-plane”. Thus, thermal stability for MRAM devices with PMA MTJs is limited since the physical law predicts thermal stability is proportional to the product of the coercive field (Hc) and the FL magnetic moment where Hc is the minimum magnetic field needed to reverse the FL magnetization direction. Also, Hc is directly related to PMA in that a larger PMA for the free layer translates to a higher Hc, and vice versa. Although greater free layer thickness provides a higher magnetic moment, the tradeoff is a reduction in PMA and coercivity. In general, there is an optimal free layer thickness for best thermal stability.
Thermal stability is a function of the perpendicular anisotropy field as shown in equation (1) where kB is the Boltzmann constant, T is the temperature, Ms is the saturation magnetization, and Hkeff,⊥ and V are the out-of-plane anisotropy field and volume, respectively, of the free layer:
                    Δ        =                                            M              S                        ⁢                          VH                                                k                  eff                                ,                ⊥                                                          2            ⁢                          k              B                        ⁢            T                                              (        1        )            
and the perpendicular anisotropy field of a magnetic layer is expressed in equation (2) as:
                              H                                    k              eff                        ,            ⊥                          =                                            -              4                        ⁢            π            ⁢                                                  ⁢                          M              s                                +                                    2              ⁢                              K                U                                  ⊥                                      ,                    s                                                                                                      M                s                            ⁢              d                                +                      H                          k              ,              χ              ,              ⊥                                                          (        2        )            
where d is the thickness of the free layer, Hk,χ,⊥ is the crystalline anisotropy field in the perpendicular direction, and KU⊥,s is the surface perpendicular anisotropy of the top and bottom surfaces of the free layer. The shape anisotropy field is represented by the term (−4πMs).
In order to improve thermal stability by increasing the value KU⊥,s, a second free layer/metal oxide interface is commonly introduced on an opposite side of the free layer with respect to the tunnel barrier. The metal oxide may be another MgO layer and is often called a cap layer or a Hk enhancing layer. Thus, a MgO/FL/MgO stack will substantially increase total PMA in the free layer thereby allowing a thicker free layer and higher thermal stability. The cap layer often contacts an uppermost MTJ layer called a hard mask, which in turn connects to a top electrode and through a top electrode array to complementary-metal-oxide-semiconductor (CMOS) units in a memory chip. Hard mask materials are typically metals or alloys such as Ta, Ru, Mo, MnPt, and their conductive oxides and nitrides as required for conventional techniques in manufacturing integrated circuits. The hard mask thickness is often greater than a total thickness of the other MTJ layers, which is generally around 100 Angstroms.
Since MTJ elements are implemented in CMOS devices, a PMA MTJ must be able to withstand annealing temperatures up to about 400° C. for 30 minutes that are commonly applied to improve the quality of the CMOS units for semiconductor purposes. It is widely recognized that interfacial PMA between adjoining CoFeB and MgO layers is optimized when both have a matching body-centered cubic (BCC) structure. During annealing, the amorphous CoFeB and MgO layers usually are transformed from an amorphous state to a BCC structure. However, PMA is easily degraded by diffusion of metal from the hard mask into the cap layer thereby interrupting the BCC formation process in the cap layer. Therefore, an improved MTJ structure is needed that enables the cap layer to achieve a pure BCC structure which in turn leads to enhanced thermal stability for the PMA MTJ at elevated temperatures up to 400° C. that are typical of back end of line (BEOL) semiconductor processes.